1. Field of the Invention
The present invention relates generally to thermoacoustic refrigerators and, more specifically, to a thermoacoustic refrigerator having a relatively small size which utilizes one or more piezoelectric drivers to generate high frequency sound within a resonator. The interaction of the high frequency sound with one or more stacks create a temperature difference across the stack which is thermally anchored at each end to a pair of heat exchangers located on opposite sides of the stack.
2. Background of the Invention
The thermoacoustic effect has a long history and it is only recently that new applications have stimulated its development. In the 18th century it was discovered that a glass tube open at one end, would produce sound when the closed end was heated. This device is known as the Soundhaus Tube. Subsequently it was discovered that a tube open at both ends will also produce sound when a metallic mesh located in the lower half of the tube is heated and the tube is held up vertically. In such a device, convection plays an important role. This is known as the Rijke Tube. It was not until the end of the 19th century when Lord Rayleigh explained how it works. The device is essentially an example of a relaxation oscillator where oscillations are sustained when energy is injected at the right phase of the oscillation cycles.
In 1975, Merkli and Thomann observed the converse of the above effect, that an acoustic field can produce cooling in a resonant tube. In 1983, Wheatley et al built the first thermoacoustic refrigerator; it operated at 500 Hz and produced temperature differences of approximately 100° C. Since the discovery by Merkli and Thomann that cooling can be produced by the thermoacoustic effect in a resonance tube, research has concentrated on developing the effect for practical applications. One approach in the art has been to increase the audio pumping rate. While the experiments of Merkli and Thomann used frequencies of around 100 Hz, Wheatley et al. successfully raised the operating frequency to around 500 Hz and achieved impressive cooling rates in their refrigerator. This has encouraged others to build various configurations of thermoacoustic refrigerators.
The essential ingredients of a thermoacoustic refrigerator or heat pump are:                i. A source of sound to pump heat into the device;        ii. A working gas, typically air at 1 atmosphere;        iii. An acoustic resonator for amplifying the level of sound and for providing phasing for the operation of the refrigerator;        iv. A secondary medium comprising a stack along which sound pumps heat, i.e. a thermal rectifier; and        v. Two heat exchangers, one at each end of stack providing a hot heat exchanger and a cold heat exchanger.        
An important element in the operation of a thermoacoustic refrigerator is the special thermal interaction of the sound field with the stack. There exists a weak thermal interaction characterized by a time constant given by ωτ≈1 where ω is the audio pump frequency and τ is the thermal relaxation time for a thin layer of gas to interact thermally with a plate or stack. The amount of gas interacting with the stack is determined approximately by the surface area of the stack and by a thermal penetration depth δk given by:δk=(2κ/ω)1/2Here κ represents the thermal diffusivity of the working fluid. By increasing ω, the weak coupling condition is met by a reduction of δk and hence of τ. The work of acoustically pumping heat up a temperature gradient as in a refrigerator is essentially performed by the gas within approximately the penetration depth. The amount of this gas has an important dependence on the frequency of the audio drive. In a high frequency refrigerator, smaller distances and masses are utilized thus making the heat conduction process relatively quick.
Each of the prior art thermoacoustic refrigerators are relatively complicated to manufacture and thus expensive. In addition, thermoacoustic refrigerators known in the art tend to be massive and typically not well suited for use on a very small level such as for use in cooling semiconductors and other small electronic devices or biological samples. Thus, it would be advantageous to provide a thermoacoustic refrigerator that can be made relatively small with a fast response time while retaining good cooling abilities. In addition, it would be advantageous to provide a thermoacoustic refrigerator that operates relatively efficiently and that is relatively simple and economical to manufacture.